A grotto (Italian grotta and French grotte) is any type of natural or artificial cave that is associated with modern, historic, or prehistoric use by humans. When it is not an artificial garden feature, a grotto is often a small cave near water and often flooded or liable to flood at high tide. The picturesque Grotta Azzurra at Capri and the grotto of the villa of Tiberius in the Bay of Naples are outstanding natural seashore grottoes. Whether in tidal water or high up in hills, they are very often in limestone geology where the acidity dissolved in percolating water has dissolved the carbonates of the rock matrix as it has passed through what were originally small fissures. See karst topography, cavern.
In modern times, many people purchase artificial grottos for ornamental and devotional purposes when it comes to placing statues of saints, particularly the Blessed Virgin, in outdoor gardens. The well-known apparitions of Our Lady of Lourdes to Bernadette Soubirous took place in a grotto, which is visited by many Catholics. Numerous garden shrines are modeled after these apparitions, and can commonly be found displayed in gardens and Churches, among other places (see Lourdes grotto).